1. Field of the Invention
The invention relates to catalysts, and more particularly, to catalysts for the removal of nitrogen oxides from exhaust gases of combustion which are contaminated with dust.
2. Description of the Prior Art
Nitrogen oxides contained in exhaust gases from combustion equipment can be converted, by catalytic reduction using ammonia, into harmless nitrogen and water. For this purpose, the exhaust gases are customarily conducted over bulk catalysts with the simultaneous introduction of gaseous ammonia, liquid ammonia or ammonia dissolved in water.
If the exhaust gases contain large quantities of dust or soot, then the bulk catalyst used quickly becomes stopped up. If the exhaust gases also contain catalyst poisons, such as sulfur or chlorine compounds, then these substances deactivate the catalytically-active surfaces. This is especially true if catalysts are used which are sensitive to catalyst poisons.
Active catalyst substances and carriers which withstand the various catalyst poisions are expensive, and furthermore, such substances are difficult to dispose of because of their heavy metal content. Catalyst material which is easily disposed of, for example, in a blast furnace, that is, iron oxides, such as Fe.sub.2 O.sub.3, have tendencies toward the above-mentioned catalyst poisoning.
Overall, therefore, dust and catalyst poisons cause the active surface regions of the catalyst to become blocked up quickly, so that the activity of the catalyst is reduced, although there is still active catalyst material below the deactivated surface layer of the catalyst.
Some examples of catalytic technology are found in U.S. Pat. No. 4,259,312, entitled "Process And Apparatus For Catalytically Reacting A Reducing Gas And Water Vapor"; U.S. Pat. No. 4,235,604, entitled "Method For Processing Coke Oven Gas"; and U.S. Pat. No. 4,049,777, entitled "Method Of Waste Gas Treatment", all of which are assigned to at least one of the same assignees as the instant application.
Further examples of catalytic technology are found in U.S. Pat. No. 4,448,895, entitled "Process For Preparation Of Catalyst For Cleaning Exhaust Gases And Catalyst Prepared For The Process", and U.S. Pat. No. 4,503,162, entitled "Catalyst Systems For The Conversion Of Gases". All of the above-cited patents are incorporated herein by reference.